Internal combustion engines convert chemical energy from a fuel into mechanical energy. The fuel may be petroleum-based (gasoline or diesel), natural gas, a combination thereof, or the like. Some internal combustion engines, such as gasoline engines, inject an air-fuel mixture into one or more cylinders for ignition by a spark from a spark plug or the like. Other internal combustion engines, such as diesel engines, compress air in the cylinder and then inject fuel into the cylinder for the compressed air to ignite. A diesel engine may use a hydraulically activated electronically controlled unit injection (HEUI) system or the like to control the fuel injection into the cylinders. The ignited fuel generates rapidly expanding gases that actuate a piston in the cylinder. Each piston usually is connected to a crankshaft or similar device for converting the reciprocating motion of the piston into rotational motion. The rotational motion from the crankshaft may be used to propel a vehicle, operate a pump or an electrical generator, or perform other work. The vehicle may be a truck, an automobile, a boat, or the like.
Many internal combustion engines use exhaust gases to reduce the production of nitrogen oxides (NOX) during the combustion process in the cylinders. These internal combustion engines typically mix a portion of the exhaust gases with the intake air for combustion in the cylinders. The exhaust gases usually lower the combustion temperature of the fuel below the temperature where nitrogen combines with oxygen to form NOX.
There are various approaches for mixing the exhaust gases with the intake air in an internal combustion engine. Some internal combustion engines control the opening and closing of exhaust and intake valves in a cylinder. The opening and closing of the valves may trap and push some exhaust gases from the cylinder into the intake manifold for mixing with the intake air. Other internal combustion engines use an exhaust gas recirculation (EGR) system to divert a portion of the exhaust gases exiting the cylinders for mixing with the intake air to the cylinders.
Many EGR systems divert a portion of the exhaust gases from the exhaust manifold to the intake manifold of the engine. The exhaust manifold generally is an accumulation chamber above the cylinders that gathers the exhaust gases for expulsion from the vehicle. The intake manifold generally is another chamber above the cylinders that holds a combustion gas for the cylinders. The combustion gas may be all intake air or a combination of intake air and exhaust gases. The amount of exhaust gases in the combustion gas may vary during engine operation. The internal combustion engine may have a by-pass pipe to supply intake air directly to the intake manifold without exhaust gases.
EGR systems usually have an EGR conduit or pipe connected to the exhaust manifold. The EGR conduit may be a channel formed by the cylinder head or other engine component, a pipe or tube outside the cylinder head, a combination thereof, or the like. The EGR conduit may direct the exhaust gases through a gas trap and a gas cooling device prior to mixing the exhaust gases with the intake air. The gas trap usually is cleaning device for removing particulate from the exhaust gases. The gas cooling device may be a heat exchanger or other device for removing heat from the exhaust gases. The gas cooling device may use coolant from the engine cooling system, a separate cooling system, or a combination thereof. Some EGR systems have an orifice or other pressure measurement device to measure the exhaust gas flow through the EGR conduit.
Many EGR systems may have a control valve connected between the EGR conduit and the exhaust manifold. The engine controller or another microprocessor usually activates the control valve to adjust the flow of exhaust gases through the EGR conduit to achieve a selected concentration of exhaust gases in the intake air. The selected concentration of exhaust gases may vary during engine operation. The control valve may be actuated using a vacuum, a hydraulic fluid such as the hydraulic fluid used in fuel injectors, or the like. Some EGR systems open the control valve only when the pressure of the exhaust gases is higher than the pressure of the intake air. Some EGR systems have a valve in the exhaust duct to restrict the exhaust flow from the engine. The restricted flow increases the back pressure of the exhaust gases. The valve may open or close to control the amount of back pressure and thus may control the flow of exhaust gases into the intake air.
Many EGR systems have a mixing device at the connection of the EGR conduit with the intake conduit that supplies intake air for the cylinders. The intake conduit may be connected to the output of a compressor that pressurizes the intake air. The mixing device typically combines exhaust gases from the EGR conduit with intake air from the intake conduit to form the combustion gas for the cylinders. During engine operation, the exhaust gases usually flow into the intake air when the pressure of the exhaust gases is greater than the pressure of the intake air. The intake air pressure may vary especially when a turbocharger is used.
The mixing device typically is a pipe or other union between the EGR conduit and the intake conduit. The EGR and intake conduits may form a “tee” or similar connection. The mixing device may form a mixing chamber. The EGR conduit and/or intake conduit may expand to form the mixing chamber at the connection. The mixing device mixes the exhaust gases with the intake air to form the combustion gas. The mixing device usually supplies the combustion gas to the intake manifold through a supply pipe or conduit.
The mixing device may have an EGR conduit that extends into the intake conduit. The EGR conduit may create an obstacle that separates the intake air into two streams, each passing on an opposite side of the EGR conduit. The exhaust gas exits the EGR conduit and enters into a region of the mixing device where the two streams are essentially absent. The two intake air streams combine with each other and with the exhaust gases downstream from the EGR conduit.
The mixing device may have a venturi for combining the exhaust gases with the intake air. The venturi typically forms part of the intake conduit. The venturi usually has an inlet connected to an outlet by a nozzle or throat. The inlet and outlet have larger diameters than the throat. The diameter of the intake conduit tapers down from the inlet to the throat and then tapers up from the throat to the outlet. The EGR conduit connects to the throat. The inlet may be connected to the compressor output of a turbocharger. The outlet is connected to the supply conduit for providing the combustion gas to the intake manifold.
In operation, the venturi creates a pressure drop in the intake air passing through the throat. The smaller diameter of the throat increases the velocity of the intake air. The increase in velocity lowers the pressure of the intake air in throat. The lower pressure of the intake air increases the amount of exhaust gases that can enter the throat for mixing with the intake air to form the combustion gas. At the outlet, the larger diameter decreases the velocity of the combustion gas. The decrease in velocity increases the pressure of the combustion gas.
Many mixing devices may not adequately blend the intake air with the exhaust gases to form a combustion gas with an essentially uniform dispersion of the exhaust gases in the intake air. While the exhaust gases and intake air are combined, there may be an uneven dispersion of the exhaust gases in the intake air. The uneven dispersion may include pockets, zones, regions, or strata of higher or lower concentrations of exhaust gases than the selected concentration of exhaust gases in the intake air. The dispersion may be more uneven when the exhaust gases enter on one side of the intake air stream. The selected concentration of exhaust gases in the intake air may be reduced to avoid or reduce the effects of the uneven dispersion on engine operation. Internal combustion engines may produce more NOX at the lower selected concentrations of exhaust gases in the intake air.